Lord Heseltine, among others, was incandescent. Mr Johnson’s remarks were “preposterous and obscene”. The former London mayor “was losing the plot,” said Lord Heseltine, adding: “I think the strain is beginning to tell.”

But this was no throwaway remark. Mr Johnson, an amateur historian, who has written books on Churchill and Ancient Rome, was trying to point out the threat to British sovereignty of a European superstate, which anyway is doomed to fail.

“The truth is that the history of the last couple of thousand years has been broadly repeated attempts by various people or institutions... to rediscover the lost childhood of Europe,” he explained, pointing out Hitler, Napoleon et al had tried and failed.

Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods.Boris Johnson

“The EU is an attempt to do this by different methods,” he concluded. On Saturday, Mr Johnson’s analysis received timely backing from Andrew Roberts, the eminent Right-wing historian and author of a number of books on Hitler, the Second Word War and Napoleon.

“I think Boris Johnson is absolutely right from a historical perspective,” said Prof Roberts, “There is something inherently destabilising about trying to get 30-odd states into one political entity. This one-size fits all doesn’t work historically speaking. The Remain campaign are screaming about something that Boris didn’t say and it’s a straw man argument to argue against something that somebody didn’t say in the first place.

“Boris could have used other examples such as Philip II of Spain, Louis XIV of France or Kaiser Wilhelm II of Germany, but ultimately his remarks were absolutely right in the view of the fact he used the words 'by different methods’.”

There is certainly evidence that Mr Johnson’s analogy enjoyed a wider resonance. Lord Ashcroft, the Conservative Party’s former treasurer and deputy chairman, who has been conducting regular polling on the referendum, suggested Mr Johnson’s historical analysis chimed with two carefully selected focus groups of undecided voters, one in Birmingham and the other in Glasgow.

Lord Michael Heseltine at home in LondonCredit:
Geoff Pugh

They were aware of Mr Johnson’s incendiary interview but unclear on the detail. Undecided voters came away concluding he had made a “valid” and “persuasive” case for Brexit.

On his website lordashcroftpolls.com, Lord Ashcroft wrote: “Some of the participants knew Hitler had come into it somewhere, though some were not sure how (“he didn’t do one of those salutes, did he?”) and a few who had heard what he had said thought it had been in poor taste.

“But not for the first time, after hearing what he said in his Telegraph interview, most thought the fuss was out of all proportion to his remarks.

“The argument that the EU was trying to unite Europe by other means where various figures in history had failed to do so by force was quite a persuasive point for some (“it would make me think again”; “Hitler wanted to rule Europe and so does Angela Merkel and Luxembourg”) and certainly a valid one to make: 'He was alluding to the fact that people have tried to amalgamate Europe many times, and this is another version of it’.”

Antony Beevor, the celebrated historian and author of among others Stalingrad and D-Day: the Battle for Normandy, complains that politicians seemingly cannot resist the urge to reference the Second World War.

“It is totally wrong to make these sorts of comparisons,” he said, “Politicians feel the war is something that resounds with voters; that it is some kind of reference point for every crisis and conflict. Politicians revert to these comparisons because it makes them sound Churchillian.

Former London Mayor Boris Johnson Ministers arrive for a Cabinet meeting at 10 Downing StreetCredit:
REX/Shutterstock

“Historical parallels are dangerous and should be avoided at all costs. Boris should know better. It is a throw-away line but it’s a pretty silly one. It’s not helping the Out campaign. I would have thought it’s a great mistake.”

Not that, says Mr Beevor, the Out campaign has a monopoly on the war. Earlier this month, David Cameron invoked Sir Winston Churchill in insisting the European Union had helped to guarantee 60 years of peace since the end of hostilities in 1945.

In a keynote speech, Mr Cameron highlighted the battles of Trafalgar, Blenheim, Waterloo and the two World Wars as evidence that Britain cannot pretend to be “immune from the consequences” of events in Europe.

“The serried rows of white headstones in lovingly tended Commonwealth war cemeteries stand as silent testament to the price this country has paid to help restore peace and order in Europe,” he said. Mr Beevor is equally aghast at that assertion.

“The European Union did not save Europe from another war. It is a question of governance: democracies don’t fight each other. Europe was saved first of all by the Marshall Plan [the US programme to aid Europe recovery after the war] and then up to a degree the unification through Nato.”

Mr Johnson was well aware of the power of using Hitler in his backing of Brexit. In 1990, The Spectator, a magazine he would later edit, had carried an interview with Nicholas Ridley, at the time the trade and industry secretary in Margaret Thatcher’s Cabinet, in which Ridley had bemoaned the threat to British sovereignty.

“I’m not against giving up sovereignty in principle, but not to this lot. You might just as well give it to Adolf Hitler, frankly,” said Ridley.

Mr Johnson believes that, more than a quarter of a century on, Ridley had been absolutely right to voice those concerns. Ridley, who would later be made a lord and who died in 1993, would pay for his remarks with his job.

He wasn’t helped by a superb cartoon on the front of the magazine to accompany the interview in which the minister was depicted running away, having added a Hitler moustache to a poster of Helmut Kohl, the German Chancellor. Nor was Ridley the last politician to invoke Hitler and the EU in the same breath.

Tony Benn at the Labour Party Conference, 1991Credit:
Ken Mason

In 2007, Tony Benn, the towering figure of the Labour left, said of the European Union: “And, you know, before the war, Hitler wasn’t exactly a democrat. And that practice of everything being decided at the top has been carried on.” His son Hilary Benn, the shadow foreign secretary, was incidentally one of the first politicians out of the blocks to criticise Johnson, describing his comments as “deeply offensive”.

There are historic parallels between 1990 when Ridley was forced to resign and now. The Conservative Party was beset by rifts on Europe that would cripple John Major’s Government just as David Cameron’s has been bitterly divided on the Referendum.

Back in 1990, Mrs Thatcher was trying to resist a push for Britain to sign up to a single European currency. Even her greatest critics would argue Britain in rowing back against the European tide had saved itself from economic calamity.

Lord Heseltine remains a cheerleader for Europe while the likes of Peter Lilley, who would take Ridley’s job in Cabinet, remain vehemently opposed. The battle lines, to use a warlike metaphor, have been long entrenched. Those deep divisions might well explain Lord Heseltine’s astonishing attack on a fellow Tory.

In an interview with the BBC’s Newsnight, Lord Heseltine said: “When he [Johnson] starts invoking the memories of Hitler, that has crossed the bounds of domestic debate.

“It was about the most manic nationalist aggressive destruction on a scale unprecedented in human history. It was about the persecution of the Jews.

“The idea that a serious British politician can in any way invoke that memory, I find, frankly, I had better contain my language.”

Mr Lilley, a staunch Brexiteer, said Johnson’s referencing of Hitler was “ill advised” but added that the former London mayor’s comments – unlike Ridley’s remarks 26 years ago – were not personally offensive.

“What Boris said didn’t have any resonance with Angela Merkel or the current German government. But it is a distraction to mention Hitler,” said Mr Lilley, still a Conservative MP. “The truth is we don’t want to be ruled by people other than ourselves.”

By Wednesday of last week, David Cameron was piling in. “I just I think he’s wrong,” the prime minister told listeners to LBC radio. “Hitler wanted to snuff out democracy across the continent and the European Union is basically an alliance of countries that share a view about democracy and liberal values.”

Field Marshall Lord Bramall, a former head of the Armed Forces, described Mr Johnson’s remarks as “laughable”. “I know only too well this comparison of the EU and Nazi Germany is absurd,” said the veteran of the D-Day landings, “Hitler’s main aim was to create an empire in the East and violently subjugate Europeans.”

Brexiteers in turn rushed to Mr Johnson’s defence, bolstering support on the airwaves in the immediate aftermath of the Sunday Telegraph interview.

“I think the whole process of trying to drive Europe together by force or by bureaucracy and democratic means ultimately makes problems,” said Iain Duncan Smith, the former Conservative party leader.

Chris Grayling, the Leader of the Commons, told the BBC: “Boris is a historian and he was doing a piece of historical analysis. What Boris was making was a historical point.” Chris Bickerton, politics lecturer at Cambridge University and author of The European Union: A Citizen’s Guide, which has become an unlikely bestseller, said there was some merit to his historical analysis.

Watch | Hilary Benn: Boris' Hitler comments are 'deeply offensive'

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“Hitler, in his attempt to create a German empire, did use the union of Europe in trying to describe his project for German domination.

“In fact, federalists in the late 1940s and 50s found it very difficult to make their case for European federalism and one of the reasons was the whole idea had been tainted by its association with Hitler’s project for German domination. Where Boris has gone wrong is this idea of a growing European superstate. The last 30 years have been a nightmare for federalists. It has gone in the wrong direction.”

Whether Mr Johnson’s Telegraph interview will win the war over Europe, the country will find out on June 23.

He told The Sunday Telegraph that he believed Churchill would have been on the Brexit side. “There is no underlying loyalty to the idea of Europe,” he asserted: “It’s pretty clear to me his [Churchill’s] vision for Britain was not subsumed within a European superstate. He saw the UK as being supportive of the marriage but not a participant in the marriage – that is the crucial thing.”

He would go on to claim the euro had allowed the German economy to “destroy” weaker rivals.

“The euro has become a means by which superior German productivity is able to gain an absolutely unbeatable advantage over the whole Eurozone territory,” he asserted. Nobody would much argue with that part of Mr Johnson’s hypothesis. The allusion to Hitler may or may not be questionable. On grounds of taste, accuracy or both.

But it’s an incontrovertible fact, as Lord Ashcroft’s polling also discovered, that by dropping the “H” bomb into the Referendum row, Boris did what Boris always does: he livened up the debate no end.